Create listing of game engines with links to packaged app instructions.

chrome/common/extensions/docs/templates/json/apps_sidenav.json

 [
   {
     "title": "Discover",
     "items": [
       {
         "title": "What Are Chrome Apps?",
         "href": "/apps/about_apps"
       }
     ]
   },
@@ skipping 58 matching lines @@
                 "title": "About MVC Architecture",
                 "href": "/apps/app_frameworks"
               },
               {
                 "title": "Build Apps with AngularJS",
                 "href": "/apps/angular_framework"
               },
               {
                 "title": "Build Apps with Sencha Ext JS",
                 "href": "/apps/sencha_framework"
+              },
+              {
+                "title": "Game Engines",
+                "href": "/apps/game_engines"
               }
             ]
           }
         ]
       },
       {
         "title": "Chrome App Concepts",
         "toggleable": true,
         "items": [
           {
@@ skipping 184 matching lines @@
         "title": "Google Groups",
         "href": "https://groups.google.com/a/chromium.org/forum/#!forum/chromium-apps"
       },
       {
         "title": "Stack Overflow",
         "href": "http://stackoverflow.com/questions/tagged/google-chrome-app"
       }
     ]
   }
 ]